How to shrink an image with the Video Sequence Editor
When import image into VSE, it is true that the original aspect will be changed to match the scene resolution. To fix this, you can simply add another Transform strip to first Transform (NOT the picture strip itself).

So, basically one for Uniform scale, and another one for aspect ratio correction.







P.S.: When setting the second Transform scale, be sure to set the longest side to 1.000, which is X in this case.